Wideband Flat Lenses Based on Artificial Dielectric Layers

Abstract

Planar lenses based on metasurfaces or resonant elements are typically narrowband due to phase wrapping, which is strongly frequency-dependent. On the contrary, true-time-delay (TTD) planar lenses, which do not resort to phase wrapping, can achieve large bandwidths. One convenient way to design wideband TTD lenses is by means of artificial dielectric layers (ADLs), which are stacks of sub-wavelength-period patch arrays embedded in a host medium to increase its effective permittivity to values higher than commercially available dielectrics. The procedure to retrieve the effective refractive index for a given ADL stratification is reported, as well as the synthesis of the multi-section transformers in ADL technology. Trade-offs including bandwidth, focal ratio, lens diameter, and thickness, are discussed and related to the manufacturing constraints of artificial dielectrics, such as the number of metal layers, maximum achievable effective permittivity, and smallest features realizable in printed circuit board technology. An example of design is also presented, operating from 70 to 140 GHz with a lens diameter of 11.5 wavelengths at the highest frequency. A modeling method for the analysis of a flat lens based on ADLs is presented. This consists of a combined geometrical optics (GO) / physical optics (PO) approach, where each GO ray is studied as a plane wave problem to evaluate the reflection/transmission through each unit cell of the lens. This analysis is extended to consider the bending of the rays through the lens and propagation through multiple unit cells.